This invention relates to a projection exposure apparatus for semiconductor manufacture and, more particularly, to a projection exposure apparatus for semiconductor manufacture which is usable in a lithographic process for the production of semiconductor devices or liquid crystal display devices, for example.
The density of an integrated circuit is increasing, and thus, projection exposure apparatuses for semiconductor manufacture should have a very high resolving power for projection exposure of a wafer to a circuit pattern formed on a reticle. In projection optical systems of such projection exposure apparatuses, for improvement of the resolution, the numerical aperture (NA) has been enlarged or light of shorter wavelengths has been used. At present, with a projection exposure apparatus having a light source of a KrF excimer laser (xcex=248 nm) and NA of 0.6, a resolution of 0.18 micron is attainable.
Recently, super-resolution exposure techniques based on modified illumination such as ring-zone illumination or quadrupole illumination have been proposed. A resolution of 0.15-0.1 micron may be attainable with them.
For production of a high resolution projection optical system, it is necessary to perform precise adjustment after a projection optical system is assembled. More specifically, for a projection optical system, optical evaluations in regard to spherical aberration, coma, distortion, and exposure magnification, for example, should be done. While adjusting lens group spacings or eccentricities, the optical performance that satisfies predetermined specifications is pursued. Usually, the evaluation of optical performance is made by projecting and printing an image of a mask pattern upon a resist (photosensitive material) applied to a photosensitive substrate (wafer) and by observing, after development, a resist image formed thereon.
As an alternative method, there is a method in which wavefront aberration of a projection optical system is measured by use of an interferometer. However, this method requires use of a special apparatus.
As described above, in projection exposure apparatuses, it is necessary to check the quality of a resist image for final lens performance adjustment of a projection optical system. However, this procedure involves very complicated processes such as printing a pattern on a resist-coated wafer, developing the wafer, and observing a resist image by use of a scan type electron microscope (SEM).
Additionally, since, after the optical adjustment and evaluation, a projection optical system should be mounted on a projection exposure apparatus with its lenses and lens groups held fixed so that the performance does not change, it is very difficult to adjust a projection exposure optical system once the projection optical system is incorporated into the projection exposure apparatus. Practically, however, in wafer exposure processes, the projection optical system is influenced by irradiation with illumination light and the image performance thereof changes thereby.
Conventional projection exposure apparatuses are not equipped with any effective means for measuring wavefront aberration of a projection optical system after the same is mounted on the projection exposure apparatus. The goal for re-adjustment for image performance is, therefore, unfixed, and usually, the operation is interrupted to suppress the change.
It is accordingly an object of the present invention to provide a projection exposure apparatus by which measurement of image performance of a projection optical system, being mounted on the projection exposure apparatus, can be done easily.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.